Control field serialization / deserialization

[docelic]

I think a patch as small as this could work:

diff --git a/src/crystalizer.cr b/src/crystalizer.cr
index 03989e3..397c66a 100644
--- a/src/crystalizer.cr
+++ b/src/crystalizer.cr
@@ -7,7 +7,7 @@ module Crystalizer
   protected def self.each_ivar(object : O, &) forall O
     {% for ivar in O.instance_vars %}
       {% ann = ivar.annotation(::Crystalizer::Field) %}
-      {% unless ann && ann[:ignore] %}
+      {% unless ann && (ann[:ignore] || ann[:ignore_serialize])%}
         {% key = ((ann && ann[:key]) || ivar).id.stringify %}
         yield {{key}}, object.@{{ivar}}, Variable.new(
             type: {{ivar.type}},
diff --git a/src/deserializer/non_self_describing_object.cr b/src/deserializer/non_self_describing_object.cr
index 3694fb2..9721beb 100644
--- a/src/deserializer/non_self_describing_object.cr
+++ b/src/deserializer/non_self_describing_object.cr
@@ -15,7 +15,7 @@ struct Crystalizer::Deserializer::NonSelfDescribingObject(T)
   def set_each_ivar(&)
     {% for ivar in T.instance_vars %}
       {% ann = ivar.annotation(::Crystalizer::Field) %}
-      {% unless ann && ann[:ignore] %}
+      {% unless ann && (ann[:ignore] || ann[:ignore_deserialize])%}
         {% key = ((ann && ann[:key]) || ivar).id.stringify %}
         variable = Variable.new(
           type: {{ivar.type}},
diff --git a/src/deserializer/self_describing_object.cr b/src/deserializer/self_describing_object.cr
index 60f5719..1994dff 100644
--- a/src/deserializer/self_describing_object.cr
+++ b/src/deserializer/self_describing_object.cr
@@ -16,7 +16,7 @@ struct Crystalizer::Deserializer::SelfDescribingObject(T, N)
     case key
     {% for ivar in T.instance_vars %}
       {% ann = ivar.annotation(::Crystalizer::Field) %}
-      {% unless ann && ann[:ignore] %}
+      {% unless ann && (ann[:ignore] || ann[:ignore_deserialize]) %}
         {% key = ((ann && ann[:key]) || ivar).id.stringify %}
         when {{key}}
           raise Error.new "Duplicated field for {{T}}: #{key}" if @found[{{i}}]
@@ -41,7 +41,7 @@ struct Crystalizer::Deserializer::SelfDescribingObject(T, N)
     {% i = 0 %}
     {% for ivar in T.instance_vars %}
       {% ann = ivar.annotation(::Crystalizer::Field) %}
-      {% unless ann && ann[:ignore] %}
+      {% unless ann && (ann[:ignore] || (ann[:ignore_serialize] && ann[:ignore_deserialize])) %}
       if !@found[{{i}}]
         {% if ivar.has_default_value? %}
           pointerof(@object_instance.@{{ivar}}).value = {{ivar.default_value}}